Biofuel cell

ABSTRACT

The present invention discloses a new type of biofuel cell, based on the microbial regeneration of the oxidant, ferric ions. The bio-fuel cell is based on the cathodic reduction of ferric to ferrous ions, coupled with the microbial regeneration of ferric ions by the oxidation of ferrous ions, with fuel (such as hydrogen) oxidation on the anode. The microbial regeneration of ferric ions is achieved by chemolithotrophic microorganisms such as  Acidithiobacillus ferroxidans . Electrical generation is coupled with the consumption of carbon dioxide from atmosphere and its transformation into microbial cells, which can be used as a single-cell protein.

CROSS REFERENCE TO RELATED U.S. PATENT APPLICATION

This application claims the benefit of priority from U.S. patentapplication Ser. No. 60/482,765 filed on Jun. 27, 2003, entitled BIOFUELCELL, which application was filed in English.

FIELD OF THE INVENTION

The present invention relates to a fuel cell, and more particularly thepresent invention relates to a bio-fuel cell based on the microbialregeneration of the oxidant, ferric ions, by the process of aerobicoxidation of ferrous to ferric ions by chemolithotrophic microorganismssuch as Acidithiobacillus ferroxidans that eliminates carbon dioxidefrom the atmosphere during electricity generation.

BACKGROUND OF THE INVENTION

A major component of the development of a hydrogen economy is the widescale adoption of fuel cell technology. While there have beensignificant advances towards the application of fuel cells in everydaylife, their widespread use has not been achieved yet due in part to thehigh cost of electricity they produce, see Rose, R., Fuel Cells andHydrogen: The Path Forward, Report Prepared for the Senate of the USA,http://www.fuelcellpath.org.

The slow kinetics of the oxygen reduction reaction on the cathode of themost popular proton-exchange membrane (PEM) hydrogen-oxygen fuel cell isthe main reason for both the high cost of the fuel cell itself(requirement of Pt as catalyst) and of low electrical fuel efficiency,around 50% as disclosed in Bockris, J. O.-M. and R. Abdu, J.Electroanal. Chem., 448, 189 (1997).

The use of redox fuel cells, in which oxygen is replaced by otheroxidants, such as ferric ions, can result in the increase of the rate ofcathodic reaction (or exchange current density in electrochemical terms)by several orders of magnitude, as disclosed in Bergens, S. H., G. B.Gorman, G. T. R. Palmore and G. M. Whitesides, Science, 265, 1418(1994); Larsson, R. and B. Folkesson, J. Appl. Electrochem., 20, 907(1990); and Kummer, J. T. and D.-G. Oei, J. Appl. Electrochem., 15, 619(1985).

In addition, the rate of mass transfer of oxidant to the electrodesurface (corresponding to limiting current density in electrochemicalterms) is also higher, mainly because of the higher aqueous solubilityof the oxidant in redox fuel cells (for example, 50 g/L for Fe³⁺) ascompared to that of oxygen (between 0.006 and 0.04 g/L, depending on thepartial pressure and temperature). All these characteristics of theredox fuel cells should theoretically allow efficiencies for thetransformation of chemical to electrical energy of 80 to 90% to beachieved using non-noble metal electrodes based on thermodynamicarguments. However, the main problem in redox fuel cells is theefficiency of reoxidation of the reduced form of the oxidant (oxidantregeneration), see Larsson, R. and B. Folkesson, J. Appl. Electrochem.,20, 907 (1990); and Kummer, J. T. and D.-G. Oei, J. Appl. Electrochem.,15, 619 (1985).

For example, γ-ray irradiation has been used for the reoxidation of Fe²⁺to Fe³⁺ in a H₂—Fe³⁺/Fe²⁺ redox fuel cell as disclosed in Yearger, J. F,R. J. Bennett and D. R. Allenson, Proc. Ann. Power Sources Conf., 16, 39(1962). While the efficiency of the fuel cell itself was very high, thereported efficiency of the oxidant regeneration was well below 15%. Inother cases, regeneration of the oxidant is carried out using oxygenover expensive catalyst [see Bergens, S. H., G. B. Gorman, G. T. R.Palmore and G. M. Whitesides, Science, 265, 1418 (1994)] whicheliminates the advantage of the use of non-platinum cathode, and isstill slow.

Therefore, in order to develop a practically viable redox fuel cell withhigh overall efficiency, it is necessary to develop an efficient methodfor oxidant regeneration as suggested in Larsson, R. and B. Folkesson,J. Appl. Electrochem., 20, 907 (1990).

The process of aerobic oxidation of ferrous to ferric ions bychemolithotrophic microorganisms such as Acidithiobacillus ferroxidans(A. ferrooxidans) was discovered more than half a century ago, see A. R.Colmer, M. E. Hinkle, Science, 106 (1947) 253-256. These microorganismshave been widely used in metallurgy for the leaching of noble (Au),heavy (U) and base (Cu, Ni, Zn, Co) metals, as well as in environmentalprotection. The microbial iron oxidation is based on the following netreaction:4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O  (1)

It has been shown that the rate of microbial oxidation of ferrous ionsis 500,000 times faster than that obtained by purely chemical reactionwith oxygen at pH between 1 and 2, see D. T. Lacey, F. Lawson,Biotechnology and Bioengineering, 12 (1970) 29-50.

When growing on ferrous iron oxidation, A. ferrooxidans uses one of thenarrowest thermodynamic limits known in microbial world, see W. J.Ingledew, Biochimica et Biophysica Acta, 683 (1982) 89-117. The electrontransport chain of iron oxidation by this microorganism contains twohalf-reactions:4Fe²⁺=4Fe³⁺+4e⁻  (2)which takes place outside of the cell membrane, and4e ⁻+O₂+4H⁺=2H₂O  (3)inside of the membrane, see M. Nemati, S. T. L. Harrison, G. S.Hansford, C. Webb, Biochemical Engineering Journal, 1 (1998) 171-190.The electrons are transported through the cell wall via a chain of threeelectron carriers—rusticyanin, cytochrome c and cytochrome a.

The iron-oxidizing bacterium A. ferrooxidans is an autotrophicmicroorganism, i.e. it uses carbon dioxide (CO₂), usually fromatmosphere, as a sole source of carbon, while inorganic reactions suchas ferrous iron oxidation (1-3) supply it with energy. The laboratory-pilot- and industrial-scale oxidation of iron by A. ferrooxidans hasbeen studied in different types of bioreactors. Under the usualcultivation conditions in a bioreactor containing A. ferrooxidans grownon ferrous ions, the redox potential can reach a value of 1000 mV, seeM. Boon, K. C. A. M. Luyben, J. J. Heijnen, Hydrometallurgy, 48 (1998)1-26. Since the potential of reaction (3) is 1120 mV vs. standardhydrogen electrode (SHE), up to approx. 90% of the reaction energy isused for the production of Fe3+, while the rest (˜10%) is available tomicroorganisms for biomass formation and maintenance.

The blooxidation of ferrous iron by A. ferrooxidans has been used inelectrochemical cells for several different purposes. In all thesecases, the electrochemical reaction, taking place on the surface of thecathode is:Fe⁺ +e=Fe²⁺  (4)

Several different counter-electrode (anode) reactions have beendescribed:

A) Oxygen Formation According to the Reaction:2H₂O=4 e ⁻+O₂+4H⁺  (5a)

In that case, it is necessary to apply external electrical potential inorder to reduce the ferric iron on one electrode and to produce oxygenon the other. This system has been used for the continuous regenerationof the microbial substrate (ferrous iron) which resulted in theproduction of very high cell yields, see N. Matsumoto, S. Nakasono, N.Ohmura, H. Saiki, Biotechnology and Bioengineering, 64 (1999) 716-721;and S. B. Yunker, J. M. Radovich, Biotechnology and Bioengineering, 28(1986) 1867-1875.

B) Oxidation of Ferric Ions:

This type of electrobioreactor has been used to determine the rate ofmicrobial ferrous iron oxidation by measuring the value of theelectrical current, see H. P. Bennefto, D. K. Ewart, A. M. Nobar, I.Sanderson, Charge Field Eff. Biosyst.—2, [Proc. Int. Symp.], (1989)339-349; and K. Kobayashi, K. Ibi, T. Sawada, Bioelectrochemistry andBioenergetics, 39 (1996) 83-88.

C) Oxidation of Organic Compounds such as Methanol:CH₃OH+H₂O═CO₂+6H⁺+6e ⁻  (5c)

This system has been used for the electrochemical degradation ofpollutants (methanol) in water, see A. Lopez-Lopez, E. Exposito, J.Anton, F. Rodriguez-Valera, A. Aldaz, Biotechnology and Bioengineering,63 (1999) 79-86.

No literature data has been found describing a fuel cell for theproduction of electricity, based on the cathodic reduction of ferric toferrous ions, coupled with the microbial regeneration of ferric ions bythe oxidation of ferrous ions. The above analysis of the energetics offerrous iron oxidation by A. ferrooxidans shows that up to 90% of theGibbs energy of microbial oxygen reduction can be used for the ironoxidation, i.e. production of electricity, while the rest will beconsumed by the microorganisms for maintenance and formation of new cellbiomass. It has also been found that the growth of A. ferrooxidans canbe uncoupled from iron oxidation under certain conditions, see M.Nemati, S. T. L. Harrison, G. S. Hansford, C. Webb, BiochemicalEngineering Journal, 1 (1998) 171-190, i.e. these microorganisms canoxidize ferrous iron under zero-growth conditions.

It has been recognized that the global warming, caused mainly byanthropogenic carbon dioxide emissions, is one of the main problemswhich humanity faces at the moment. Presently, the most promising way toreduce the release of carbon dioxide to atmosphere seems to be thetransition from fossil fuel economy to hydrogen economy, see J.O.M.Bockris, International Journal of Hydrogen Energy, 27 (2002) 731-740.

Presently known oxygen/hydrogen fuel cells do not produce carbon dioxidewhen using hydrogen as fuel. However, it would be even more advantageousto provide a bio-fuel cell based on chemolithotrophic microorganismssuch as Acidithiobacillus ferroxidans which exhibit very high efficiencyand which consumes CO₂ from atmosphere during its operation.

SUMMARY OF INVENTION

An object of the present invention is to provide a redox fuel cell withan efficient method for the oxidant regeneration and which consumes CO₂.

In a preferred embodiment of the invention there is provided a bio-fuelcell based on the cathodic reduction of ferric to ferrous ions, coupledwith the microbial regeneration of ferric ions by the oxidation offerrous ions, with fuel (such as hydrogen) oxidation on the anode. Themicrobial regeneration of ferric ions is achieved by chemolithotrophicmicroorganisms such as Acidithiobacillus ferroxidans.

In one aspect of the invention there is provided a bio-fuel cell system,comprising;

a) a fuel cell including a cathode compartment containing a cathodeelectrode with an aqueous solution containing ferric ions (Fe³⁺) beingcirculated into said cathode compartment with a reaction at the cathodeelectrode being reduction of ferric ions at the cathode electrode in areaction given by 4Fe³⁺+4e⁻=4Fe²⁺;

an anode compartment containing an anode electrode with a fuel having ahydrogen constituent being pumped into said anode compartment, saidanode compartment being separated from said cathode compartment by amembrane permeable to protons, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+) formed by the oxidation of hydrogencross the proton exchange membrane into the cathode compartment; and

b) a bioreactor containing chemolithotrophic microorganisms, a pump forpumping a fluid containing oxygen (O₂) and carbon dioxide into thebioreactor, the bioreactor being in flow communication with the cathodecompartment so that the aqueous solution containing ferrous ions (Fe²⁺)and protons (H⁺) is circulated from the cathode compartment to thebioreactor where the ferrous ions (Fe²⁺) are oxidized by thechemolithotrophic microorganisms to ferric ions (Fe³⁺) in an aerobicoxidation reaction given by 4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O, wherein electricalpower is obtained by making electrical connection between a load and theanode and cathode electrodes, and including a pump for pumping a fluidcontaining ferric ions (Fe³⁺) into said cathode compartment.

The membrane permeable to protons may be a proton exchange membrane.

In another aspect of the present invention there is provided a bio-fuelcell system, comprising;

a) a cathode compartment containing a cathode electrode, a pump forpumping a fluid containing oxygen and carbon dioxide into the cathodecompartment;

b) an anode compartment containing an anode electrode with a fuel havinga hydrogen constituent being pumped into said anode compartment, saidanode compartment being separated from said cathode compartment by amembrane permeable to protons, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+), formed by the oxidation of the fuelcross, wherein protons (H+), formed by the oxidation of the fuel crossthe membrane into the cathode compartment; and

c) chemolithotrophic microorganisms immobilized on said cathodeelectrode, an aqueous solution containing substantially no iron, coatingsaid chemolithotrophic microorganisms for maintaining a suitablehumidity of the microbial cells, wherein a reaction at the cathodeelectrode is biological reduction of oxygen at the cathode electrode ina reaction given by O₂+4H++4e−=2H₂O, wherein electrons in that reactionare obtained by transfer from the cathode electrode to the attachedmicrobial cells, wherein electrical power is obtained by makingelectrical connection between a load and the anode and cathodeelectrodes.

In another aspect of the invention there is provided a bio-fuel cellsystem, comprising;

a) a fuel cell including a cathode compartment containing a cathodeelectrode, the cathode electrode including an Fe-copolymer containing aredox couple (Fe²⁺/Fe³⁺), chemolithotrophic microorganisms beingimmobilized on the Fe-copolymer, with a reaction at the cathodeelectrode being reduction of ferric ions at the cathode electrode in areaction given by 4Fe³⁺+4e⁻=4Fe²⁺, a pump for pumping a fluid containingoxygen (O₂) and carbon dioxide (CO₂) into the cathode compartment; and

b) an anode compartment containing an anode electrode with a fuel havinga hydrogen constituent being pumped into said anode compartment, saidanode compartment being separated from said cathode compartment by amembrane permeable to protons, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+), formed by the oxidation of the fuelcross the membrane into the cathode compartment, and wherein the ferrousions are oxidized by the chemolithotrophic microorganisms to ferric ions(Fe³⁺) in an aerobic oxidation reaction in the cathode compartment givenby 4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O, wherein electrical power is obtained bymaking electrical connection between a load and the anode and cathodeelectrodes.

The bioreactor and the cathode compartment may contain dissolvednutrients for facilitating growth of the chemolithotrophicmicroorganisms.

The present invention also provides a bio-fuel cell system, comprising;

a) a cathode compartment containing a cathode electrode,chemolithotrophic microorganisms being immobilized on the cathode and incontact with an aqueous solution containing a salt of iron, with areaction at the cathode electrode being reduction of ferric ions at thecathode electrode in a reaction given by 4Fe³⁺+4e⁻=4Fe²⁺, a pump forpumping a fluid containing oxygen (O₂) and carbon dioxide into thecathode compartment; and

b) an anode compartment containing an anode electrode with a fuel havinga hydrogen constituent being pumped into said anode compartment, saidanode compartment being separated from said cathode compartment by amembrane permeable to protons, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+) formed by the oxidation of the fuelcross the membrane into the cathode compartment, and wherein the ferrousions (Fe²⁺) are oxidized by the chemolithotrophic microorganisms toferric ions (Fe³⁺) in an aerobic oxidation reaction in the cathodecompartment given by 4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O, wherein electrical poweris obtained by making electrical connection between a load and the anodeand cathode electrodes.

In another aspect of the invention there is provided a method forgenerating electricity, comprising;

a) pumping a fluid containing oxygen and carbon dioxide into a cathodecompartment of a fuel cell, the cathode compartment including a cathodeelectrode and a redox couple present therein with a reaction at thecathode electrode being reduction of a first member of the redox coupleto a second member of the redox couple in a lower oxidation state;

b) pumping fuel into an anode compartment of the fuel cell containing ananode electrode with the fuel having a hydrogen constituent, said anodecompartment being separated from said cathode compartment by a protonexchange membrane, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+), formed by the oxidation of the fuelcross the proton exchange membrane into the cathode compartment; and

c) oxidizing the second member of the redox couple in the loweroxidation state back to the higher oxidation state by chemolithotrophicmicroorganisms in the presence of oxygen wherein electrical power in anelectrical load is obtained by making electrical connection between theelectrical load and the anode and cathode electrodes.

The bio-fuel cell system may be used for production of biomass bypumping carbon dioxide (CO₂) and oxygen into the chamber containingchemolithotrophic microorganisms. Thus both oxygen and carbon dioxidemay be pumped into the fuel cell by pumping air into the apparatus.

Controlling a ratio of electrical production to biomass production canbe achieved by varying microbial cultivation parameters including anelectrical potential of the cathode electrode, or by varying the ratioof Fe²⁺/Fe³⁺ concentrations, or a combination of both.

The chemolithotrophic microorganisms may be Acidithiobacillusferroxidans.

BRIEF DESCRIPTION OF DRAWINGS

The following is a description, by way of example only, of the biofuelcell constructed in accordance with the present invention, referencebeing had to the accompanying drawings, in which:

FIG. 1 shows a diagrammatic representation of a bio-fuel cellconstructed in accordance with the present invention;

FIG. 2 is a plot of cathode potential versus current density achievedwith the fuel cell of FIG. 1;

FIG. 3 is a plot of cathode potential versus oxidant flow rate into thecathode compartment of the fuel cell of FIG. 1;

FIG. 4 is a plot of fuel cell potential versus oxidant flow rate intothe cathode compartment of the fuel cell of FIG. 1;

FIG. 5 is a plot of cathode potential versus time for extended operationof the fuel cell of FIG. 1;

FIG. 6 a shows an alternative embodiment of a bio-fuel cell constructedin accordance with the present invention;

FIG. 6 b shows an alternative embodiment of the bio-fuel cell of FIG. 6a;

FIG. 7 a shows another alternative embodiment of a bio-fuel cell;

FIG. 7 b an alternative embodiment of the bio-fuel cell of FIG. 7 a;

FIG. 8 a shows another alternative embodiment of a bio-fuel cell; and

FIG. 8 b an alternative embodiment of the bio-fuel cell of FIG. 8 a.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of a bio-fuel cell constructed in accordance withthe present invention is based on the microbial oxidation of ferrousions for the regeneration of the oxidant (ferric ions) in the fuel cellwhere the ferric iron are regenerated by Acidithiobacillus ferrooxidans(A. ferrooxidans) according to the reaction (1) above. Referring to FIG.1, a bio-fuel cell-bioreactor system shown generally at 10 includes afuel cell section 12 including a cathodic compartment 14 and an anodiccompartment 16 separated by a membrane 18, such as for example a Nafionproton-exchange membrane. The anode 20 may be platinized carbon felt,while the cathode 22 may be a layer of carbon felt or some other inertmaterial having a porous or otherwise high surface area. While themembrane is preferably a proton exchange membrane (PEM) other types ofmembranes may be used for separating physically the liquid in thecathode chamber from the gas (for example, hydrogen fuel) in the anodicspace. For example, the membrane does not necessarily need to be aproton-exchange membrane, but may also be an inert membrane (plastic orinorganic material) with very fine pores (less than a micrometer), whichjust separates physically the anode and cathode spaces. Non-limitingexamples include nitrocellulose membranes with a pore size below 0.2micrometers; dialysis membranes; reverse osmosis membranes.

The cathode electrode is made from a chemically inert electricallyconducting material such as carbon, nickel and stainless steel. It willbe understood that the cathode may contain a catalyst which may be oneof several catalysts, including minute amounts of gold, platinum, lead,palladium or other catalysts known to those skilled in the art.

A bioreactor 26 is in flow communication with the fuel cell section 12.A suitable bioreactor 26 which may be used has been disclosed in D. G.Karamanev, C. Chavarie, R. Samson, Biotechnology and Bioengineering, 57(1998) 471-476 which discloses a design combining an airlift system anda fibrous immobilized microbial cell support. It had a total volume of2.2 liters. In some embodiments, a inverse fluidized bed biofilm reactormay be used as disclosed in D. G. Karamanev, L. N. Nikolov,Environmental Progress, 15 (1996) 194-196.

The bioreactor 26 is used for the highly efficient oxidation of ferrousiron ions to ferric iron ions, i.e., for the oxidant regeneration. Bydefinition, a bioreactor is a vessel in which microorganisms grow andperform biochemical reactions, such as in the present case ferrous ironoxidation. In studies to demonstrate the efficacy of the present biofuelcell, the bioreactor 26 was inoculated with A. ferrooxidans (10% v/v)isolated from a copper mine. The culture media was an aqueous solutioncontaining 0.4 M ferrous ions as sulphate and the nutrient saltcomposition of Silverman and Lundgren having a pH of 1.8. Air with aflow rate of 200 L/h was sparged into the bioreactor 26 as a source ofboth oxygen and CO₂. After the cells of A. ferooxidans werespontaneously immobilizated on the surface of the fibrous support, theoxidation of ferrous ions was observed with a rate of 1.2 g per literbioreactor volume per hour. Once 99% of ferrous ions in the bioreactormedia were oxidized, the latter was circulated with a flow rate of 90mL/h, using a peristaltic pump, through the cathodic compartment 14 ofthe fuel cell 10. The anodic compartment 16 was supplied with hydrogenat a rate of 0.3 mL/s, using a peristaltic pump (Cole-Parmer).

All the liquids which contact microrganisms (capillary or bulk, in thebioreactor or in the cathode compartment, containing iron or not) shouldalso contain one or more dissolved nutrient salts to facilitatemicrobial growth. Preferred nutrient salts include: ammonium sulfate,potassium phosphate, magnesium sulfate, potassium chloride, calciumnitrate, calcium chloride, sulfuric acid. A typical composition of thesesalts is given by Silverman and Lundgren (J. of Bacteriology, v.77, p.642 (1959)).

The oxidation reaction of hydrogen at the anode:2H₂=4H⁺+4e ⁻  (6)is coupled with the reduction of ferric ions at the cathode:4Fe³⁺+4e ⁻=4Fe²⁺  (7)The protons (H+), formed by reaction (6), cross the proton-conductingsolid electrolyte 18 into the cathode compartment 14. The ferrous ions(Fe²⁺), formed at the cathode, together with protons, are pumped to abioreactor, where they are oxidized by microorganisms to ferric ions(Fe³⁺) according to reaction (1), and then returned to the cathodecompartment 14 of the fuel cell for the next cycle of electricityproduction. The overall reaction (chemical plus biochemical) takingplace in the biofuel cell 10, can be obtained by summing the reactions1, 6 and 7 which gives:2H₂+O₂=2H₂O  (8)

Therefore, the overall reaction in the biofuel cell 10 is the same asthat in a hydrogen-oxygen fuel cell. The microorganisms plus the ironions simply act as biocatalyst, which greatly increases the rate of thecathodic reaction. The ratio between the amount of energy used forelectricity production and the amount of energy used for microbialgrowth can be easily controlled by varying cultivation conditions suchas the ferric-to-ferrous iron concentration ratio in the bioreactoreffluent. It is even possible to bring this ratio to infinity byuncoupling the microbial growth from ferrous iron oxidation. In thatcase no CO₂ is consumed and no biomass is produced.

Therefore, under ideal conditions (no energy loses in the cell), up to90% of the Gibbs free energy of reaction (8) can be used for productionof electricity while the remaining 10% will be used by microorganismsfor CO₂ fixation resulting in biomass formation, as well as for cellmaintenance. As mentioned above, the current fuel cells working onhydrogen and oxygen and using platinum as catalyst at both electrodes,have around 50% current efficiency. The rest is released as heat, whichis often difficult to utilise. Using the same fuel and oxidant, the newbiofuel cell will produce electricity and microbial mass.

Since the cathodic reaction (7) on a carbon electrode is much fasterthan oxygen reduction on a platinum electrode, and since the oxygenreduction rate is the limiting factor in the currently used fuel cells,the fuel cell disclosed herein will drastically improve both the economyand environmental effect of fuel cell operation due to the 1) increasein the current efficiency; 2) elimination the use of Pt at the cathode;3) removal of carbon dioxide from atmosphere; and 4) production ofpotentially highly useful product, single-cell protein.

It has already been shown that A. ferrooxidans contains 44% protein, 26%lipids, 15% carbohydrates and at least two B-vitamins, see Tributsch, H,Nature, 281, 555 (1979). No negative physiological effect of this typeof biomass are known, see Tributsch, H, Nature, 281, 555 (1979), butobviously, more research in this direction is needed.

Studies to characterize the bio-fuel cell 10 were conducted and forthese all the potentials are given vs. the standard hydrogen electrode(SHE). The potentials were measured using an Orion pH-mV meter.

The bioreactor containing immobilized A. ferrooxidans was used tooxidize ferrous ions in batch regime. After reaching about 99%conversion of ferrous iron oxidation, the liquid phase was pumped fromthe bioreactor 26 to the cathode compartment 14 of the fuel cell. Therelationship between the cathode potential and the current density isshown in FIG. 2. The total iron concentration was 0.4 M and pH was 1.8.It can be seen that while there was some drop in the cathode potential,it was 150 mV at a current density of 35 mA/cm². This potential drop wassimilar, and in some cases, smaller than that reported in literature onthe electrochemical oxygen reduction on platinum.

The effect of the flow rate of liquid in the cathode compartment wasalso studied. The flow rate was varied between 0 and 4.2 mL/s. Twodifferent electrical loads were used—0 and 5 Ohms. The results with noelectrical load (0 Ohm) are shown in FIG. 3. It can be seen that thereis only a small increase in the cell potential, from 610 mV to 661 mV,or less than 9%. All of the potential increase was due to the cathode,and no effect of the oxidant flow rate on the anode potential wasobserved (FIG. 3), which was expected. Theoretically, the flow rateshould have no effect on the cell potential at zero load. The smallvariation (9%) observed is most probably due to the cross-current. Theeffect of the oxidant flow rate on the fuel cell voltage was alsostudied at a load of 5 Ohms. The results (FIG. 4) show that the effectis more significant than in the case of 0 Ohm load. When the flow ratewas increased from 0.5 to 3.4 mL/s, the total cell voltage firstincreased sharply, and then leveled off. The total increase was 30%.These results show that there are some mass transfer limitations of theoxidant at lower flow rates, below 2 mL/s. At flow rates, higher thatthis value, no mass transfer limitation was observed.

The stability of the biofuel cell during several hours of operation wasalso studied. It was found out (FIG. 5) that the voltage-currentcharacteristics did not change significantly during a period of 3.5hours.

Advantageously, in addition to producing electricity, the fuel cell,shown in FIG. 1, is unique in that it transforms CO₂ into cellularbiomass. Therefore, the fuel cell consumes CO₂ from atmosphere duringits operation and produces microbial mass, which can be used assingle-cell protein (SCP). It has already been shown that A. ferroxidanscontains 44% protein, 26% lipids, 15% carbohydrates and at least twoB-vitamins, see Tributsch, H, Nature, 281, 555 (1979), which makes itpotentially an excellent animal feed. No negative physiological effectof this type of biomass are known as discussed by Tributsch. It shouldbe noted that the produced single-cell protein is practically free fromboth toxic chemicals and pathogens. In the present SCP technologies,toxic chemicals can be found in the case when methanol is used as asubstrate, see Ravindra, A. P., Biotech. Adv., 18, 459 (2000). Microbialcontamination (which is sometimes toxic) is eliminated in our technologybecause there are no known pathogenic microorganisms growing oncompletely inorganic medium containing high concentrations of ironsulfate at pH between 1 and 2. The microbial contamination is a problemin many of the present methods for SCP production as discussed inRavindra, A. P., Biotech. Adv., 18, 459 (2000).

The biofuel cell system 10 of FIG. 1 requires streams of hydrogen,oxygen and carbon dioxide. As a result of the electrobiochemicalreactions, the biofuel cell produces electrical energy, heat, water (asvapour) and microbial cell mass. The hydrogen is injected into theanodic compartment of the fuel cell, while the oxygen and CO₂ areconsumed and water and the biomass are produced in the bioreactor. Inthe industrial ferrous iron oxidation bioreactors, oxygen and carbondioxide are supplied from the atmosphere.

The biofuel cell has the following characteristics, calculated on thebasis of the mass balance, stoichiometry and kinetics: During thegeneration of 100 kW of electrical energy: 4 kg/h H₂ and 4 kg/h CO₂ areconsumed; 9 kg/h biomass (SCP) are produced; and 10 m³ bioreactor ispreferred. The major advantages of the proposed biofuel cell to thecurrently known types of fuel cells are: 1) high efficiency (80-90% vs.50%, respectively); no need for noble-metal cathodes; and the uniquefeature of the biofuel cell is the consumption of carbon dioxide duringits operation production of potentially highly useful product,single-cell protein (SCP).

The energy released by the overall chemical reaction 2H₂+O₂=2H₂O is usedfor the formation of three products: electricity generation, single cellprotein (SCP) biomass production and heat generation. It is possible tooperate the fuel cell in such a manner that the ratio between theproduction of electricity and production of SCP be set at any valuebetween 0 and infinity, i.e. between “production of only biomass and noelectricity” and “production of no biomass and only electricity”. Theelectrons required for the SCP production can come either from thereaction 2H₂+O₂=2H₂O, or directly from electrical current(microorganisms consume electrons from a cathode). The ratio ofSCP/electricity can be controlled by either varying the potential of thecathode or by varying the cultivation conditions such as the ratio ofFe^(2+/)Fe³⁺ concentrations.

The embodiment of the bio-fuel cell 10 shown in FIG. 1 is such that theoxidation of ferrous iron is performed in an external bioreactor; andthe reduced form of iron (ferrous ions) is pumped back to thebioreactor. In an alternative embodiment the bioreactor and the cathodecompartment may be incorporated together as one unit. The microorganismsare immobilized on the surface of the cathode (such as carbon fibre).They are surrounded by capillary solution, containing iron salts (forexample, iron sulfates, iron chlorides, iron nitrates and theircomplexes to mention just a few. A gas containing oxygen and carbondioxide (air) is pumped to the cathode compartment space. The airdissolves in the liquid, thus supplying the microorganisms with oxygenand carbon dioxide. The scheme of this version of the biofuel cell isshown in FIG. 6 a.

FIG. 6 b shows a variation of the embodiment of FIG. 6 a in which themicroorganisms are attached to the cathode and cathode chamber isflooded with an aqueous solution containing an iron salt whichcirculates between the cathode chamber and a vessel, where O₂ and CO₂are dissolved in it by gas (air) bubbling or other means such asspraying the liquid.

In another embodiment, shown in FIG. 7 a, the redox couple (Fe2+/Fe3+)is in a solid form. The microorganisms are immobilized on the surface ofan insoluble electron-exchanger such as redox polymer, which covers thesurface of the cathode. Exemplary non-limiting Fe-copolymers that may beused are disclosed in A. Aoki and T. Miyashita, J. Electroanal. Chem.,473, 125-131 (1999).

FIG. 7 b shows a variation of the embodiment of FIG. 7 a in which againthe microorganisms are immobilized on the surface of an insolubleelectron-exchanger such as redox polymer, which covers the surface ofthe cathode and an aqueous solution circulates between the cathodechamber and a vessel, where O₂ and CO₂ are dissolved in the aqueoussolution by gas (air) bubbling or other means such as spraying theliquid.

In another embodiment of the bio-fuel cell, shown in FIG. 8 a, themicrobial cells are attached directly on the surface of the cathode, andthere is no redox couple. The cells draw electrons directly from thecathode. The humidity of the microbial cells is maintained by acapillary layer of an inorganic salt solution, such as that disclosed inSilverman, M. P. and D. G. Lundgren, J. Bacteriol., 77, 642 (1959),containing no iron salts.

FIG. 8 b shows a variation of the embodiment of FIG. 7 a in which againthe microorganisms are immobilized on directly on the surface of thecathode and an aqueous solution circulates between the cathode chamberand a vessel, where O₂ and CO₂ are dissolved in the aqueous solution bygas (air) bubbling or other means such as spraying the liquid. In thecase when microorganisms are immobilized on the cathode, it is necessaryto add an inert substance which promotes the microbial immobilization.Example: powder or gel of silicon dioxide; aluminum oxide;water-insoluble silicates; calcium sulfate, just to mention a few.

It will be understood that the present invention is not restricted toonly gaseous hydrogen/oxygen fuel cells using gaseous hydrogen fuel butmay use other hydrogen containing fuels which can undergoelectrochemical oxidation, for example methanol, ethanol, methane tomention just a few. For example, the anodic reaction in the case ofmethanol fuel is:CH₃OH+H₂O═CO₂+6H⁺+6e−

The hydrogen ions again cross the membrane, and the rest of the fuelcell, as well as the biofuel cell system is the same as in the case ofbiofuel cell using gaseous H₂ fuel.

The anodic reaction of methane as a fuel is:CH₄+O₂═CO₂+4H⁺+4e−

In the case of ethanol as a fuel, the anodic reaction is:C ₂H₅OH+3H₂O=2CO₂+12H⁺+12e−

Thus in alternative embodiments of the biofuel cell, the fuel may be acompound having a hydrogen constituent (either the only constituent inthe case of hydrogen gas or one of several constituents in the case of acompound) and electrochemical oxidation of the fuel produces protons andelectrons as with the oxidation of hydrogen but may include otherproducts as well, and the fuel is pumped into the anode compartment in afluid which may be in the form of a gas or liquid.

In addition, while a preferred chemolithotrophic microorganism for usein the biofuel cell disclosed herein is Acidithiobacillus ferroxidans itwill be understood that other microorganisms may be used, for exampleLeptospirillum ferrooxidans, Acidimicrobium, Alicyclobacillus andSulfobacillus, to mention just a few. These microorganisms work insubstantially the same way as Acidithiobacillus ferroxidans. Othermicroorganisms which work in the same way will be known to those skilledin the art and are contemplated by the inventor to be useful in thepresent invention.

As used herein, the terms “comprises”, “comprising”, “including” and“includes” are to be construed as being inclusive and open ended, andnot exclusive. Specifically, when used in this specification includingclaims, the terms “comprises”, “comprising”, “Including” and “includes”and variations thereof mean the specified features, steps or componentsare included. These terms are not to be interpreted to exclude thepresence of other features, steps or components.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

1. A bio-fuel cell system, comprising; a) a fuel cell including acathode compartment containing a cathode electrode with an aqueoussolution containing ferric ions (Fe³⁺) being circulated into saidcathode compartment with a reaction at the cathode electrode beingreduction of ferric ions at the cathode electrode in a reaction given by4Fe³⁺+4e⁻=4Fe²⁺; an anode compartment containing an anode electrode witha fuel having a hydrogen constituent being pumped into said anodecompartment, said anode compartment being separated from said cathodecompartment by a membrane permeable to protons, a reaction at the anodeelectrode being electrochemical oxidation of the fuel to produceelectrons (e⁻) and protons (H⁺), wherein protons (H+) formed by theoxidation of hydrogen cross the proton exchange membrane into thecathode compartment; and b) a bioreactor containing chemolithotrophicmicroorganisms, a pump for pumping a fluid containing oxygen (O₂) andcarbon dioxide into the bioreactor, the bioreactor being in flowcommunication with the cathode compartment so that the aqueous solutioncontaining ferrous ions (Fe²⁺) and protons (H⁺) is circulated from thecathode compartment to the bioreactor where the ferrous ions (Fe²⁺) areoxidized by the chemolithotrophic microorganisms to ferric ions (Fe³⁺)in an aerobic oxidation reaction given by 4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O,wherein electrical power is obtained by making electrical connectionbetween a load and the anode and cathode electrodes, and including apump for pumping a fluid containing ferric ions (Fe³⁺) into said cathodecompartment.
 2. The bio-fuel cell system according to claim 1 whereinthe membrane permeable to protons is a proton exchange membrane.
 3. Thebio-fuel cell system according to claim 1 wherein the membrane permeableto protons is made of a substantially inert material having poresextending therethrough less than about 10 micrometers in diameter. 4.The bio-fuel cell system according to claim 1, wherein the bioreactorand the cathode compartment contain dissolved nutrients for facilitatinggrowth of the chemolithotrophic microorganisms.
 5. The bio-fuel cellsystem according to claim 4 wherein the dissolved nutrients is one ormore of ammonium sulfate, potassium phosphate, magnesium sulfate,potassium chloride, calcium nitrate, calcium chloride and sulfuric acid.6. The bio-fuel cell system according to claim 1, wherein the fuelhaving a hydrogen constituent is selected from the group consisting ofhydrogen gas, methanol, methane and ethanol.
 7. The bio-fuel cell systemaccording to claim 1, wherein the fuel having a hydrogen constituent ishydrogen gas (H₂), and wherein the electrochemical oxidation reaction isoxidation of hydrogen at the anode electrode in a reaction given by2H₂=4H⁺+4e⁻, and so that an overall bio-fuel cell reaction is given by2H₂+O₂=2H₂O.
 8. The bio-fuel cell system according to claim 1, whereinthe chemolithotrophic microorganisms are Acidithiobacillus ferroxidans.9. The bio-fuel cell system according to claim 1, wherein thechemolithotrophic microorganisms are selected from the group consistingof Leptospirillum ferrooxidans, Acidimicrobium, Alicyclobacillus, andSulfobacillus.
 10. The bio-fuel cell system according to claim 1,wherein the cathode electrode is made from a chemically inertelectrically conducting material.
 11. The bio-fuel cell system accordingto claim 10 wherein the cathode electrode includes a layer of a porousmaterial selected from the group consisting of carbon, nickel andstainless steel.
 12. The bio-fuel cell system according to claim 10wherein the cathode electrode includes a solid plate of a materialselected from the group consisting of carbon, nickel and stainlesssteel.
 13. The bio-fuel cell system according to claim 10, wherein thecathode electrode includes a catalyst.
 14. The bio-fuel cell systemaccording to claim 13 wherein the catalyst is one of gold, platinum,palladium and lead.
 15. The bio-fuel cell system according to claim 1,wherein the bioreactor is a vessel in flow communication with thecathode compartment and enclosing the chemolithotrophic microorganisms,and wherein the aqueous solution containing ferric ions (Fe³⁺) iscirculated into said cathode compartment, including a pump forcirculating the aqueous solution containing ferrous ions (Fe²⁺) andprotons (H⁺) produced in the cathode compartment between the cathodecompartment and the bioreactor, where the ferrous ions (Fe²⁺) areoxidized by the chemolithotrophic microorganisms to ferric ions (Fe³⁺)in said aerobic oxidation reaction, and wherein the ferric ions arerecirculated back to the cathode compartment.
 16. The bio-fuel cellsystem according to claim 1 wherein the fluid containing oxygen (O₂)pumped into the bioreactor includes carbon dioxide (CO₂) for productionof biomass.
 17. The bio-fuel cell system according to claim 16 includingvoltage control means for applying and controlling a voltage on thecathode electrode for controlling a ratio of electrical production tobiomass production by varying microbial cultivation parameters.
 18. Thebio-fuel cell system according to claim 16 including reagent controlmeans for controlling a ratio of Fe²⁺/Fe³⁺ concentrations for varyingmicrobial cultivation parameters in order to control a ratio ofelectrical production to biomass production.
 19. The bio-fuel cellsystem according to claim 4 including reagent control means forcontrolling concentrations of the dissolved nutrients concentrations forvarying microbial cultivation parameters in order to control a ratio ofelectrical production to biomass production.
 20. A bio-fuel cell system,comprising; a) a cathode compartment containing a cathode electrode, apump for pumping a fluid containing oxygen and carbon dioxide into thecathode compartment; b) an anode compartment containing an anodeelectrode with a fuel having a hydrogen constituent being pumped intosaid anode compartment, said anode compartment being separated from saidcathode compartment by a membrane permeable to protons, a reaction atthe anode electrode being electrochemical oxidation of the fuel toproduce electrons (e⁻) and protons (H⁺), wherein protons (H+), formed bythe oxidation of the fuel cross, wherein protons (H+), formed by theoxidation of the fuel cross the membrane into the cathode compartment;and c) chemolithotrophic microorganisms immobilized on said cathodeelectrode, an aqueous solution containing substantially no iron, coatingsaid chemolithotrophic microorganisms for maintaining a suitablehumidity of the microbial cells, wherein a reaction at the cathodeelectrode is biological reduction of oxygen at the cathode electrode ina reaction given by O₂+4H++4e−=2H₂O, wherein electrons in that reactionare obtained by transfer from the cathode electrode to the attachedmicrobial cells, wherein electrical power is obtained by makingelectrical connection between a load and the anode and cathodeelectrodes.
 21. The bio-fuel cell system according to claim 20 whereinthe chemolithotrophic microorganisms are immobilized on said cathodeelectrode containing a substantially chemically inert material, whichfacilitates microbial immobilization.
 22. The bio-fuel cell systemaccording to claim 21 wherein the chemically inert material is one ofsilicon dioxide powder or gel, aluminum oxide (alumina) and calciumsulfate.
 23. The bio-fuel cell system according to claim 20, wherein theaqueous solution in contact with the chemolithotrophic microorganisms isa capillary layer coating the chemolithotrophic microorganisms andcathode, and wherein the fluid containing oxygen (O₂) and carbon dioxidepumped into the cathode compartment is oxygen-containing gas such asair.
 24. The bio-fuel cell system according to claim 20, wherein thefluid containing oxygen (O₂) and carbon dioxide pumped into the cathodecompartment is the aqueous solution containing oxygen (O₂) and carbondioxide dissolved therein.
 25. The bio-fuel cell system according toclaim 20, wherein the membrane permeable to protons is a proton exchangemembrane.
 26. The bio-fuel cell system according to claim 20, whereinthe membrane permeable to protons is made of a substantially inertmaterial having pores extending therethrough less than about 10micrometers in diameter.
 27. The bio-fuel cell system according to claim20, wherein the bioreactor and the cathode compartment contain dissolvednutrients for facilitating growth of the chemolithotrophicmicroorganisms.
 28. The bio-fuel cell system according to claim 20,wherein the dissolved nutrients is one or more of ammonium sulfate,potassium phosphate, magnesium sulfate, potassium chloride, calciumnitrate, calcium chloride and sulfuric acid.
 29. The bio-fuel cellsystem according to claim 20, wherein the fuel having a hydrogenconstituent is selected from the group consisting of hydrogen gas,methanol, methane and ethanol.
 30. The bio-fuel cell system according toclaim 20, wherein the fuel having a hydrogen constituent is hydrogen gas(H₂), and wherein the electrochemical oxidation reaction is oxidation ofhydrogen at the anode electrode in a reaction given by 2H₂=4H⁺+4e⁻, andso that an overall bio-fuel cell reaction is given by 2H₂+O₂=2H₂O. 31.The bio-fuel cell system according to claim 20, wherein thechemolithotrophic microorganisms are Acidithiobacillus ferroxidans. 32.The bio-fuel cell system according to claim 20, wherein thechemolithotrophic microorganisms are selected from the group consistingof Leptospirillum ferrooxidans, Acidimicrobium, Alicyclobacillus, andSulfobacillus.
 33. The bio-fuel cell system according to claim 20,wherein the cathode electrode is made from a chemically inertelectrically conducting material.
 34. The bio-fuel cell system accordingto claim 33 wherein the cathode electrode includes a fibrous layer of amaterial selected from the group consisting of carbon, nickel andstainless steel.
 35. The bio-fuel cell system according to claim 33wherein the cathode electrode includes a solid plate of a materialselected from the group consisting of carbon, nickel and stainlesssteel.
 36. A bio-fuel cell system, comprising; a) a fuel cell includinga cathode compartment containing a cathode electrode, the cathodeelectrode including an Fe-copolymer containing a redox couple(Fe²⁺/Fe³⁺), chemolithotrophic microorganisms being immobilized on theFe-copolymer, with a reaction at the cathode electrode being reductionof ferric ions at the cathode electrode in a reaction given by4Fe³⁺+4e⁻=4Fe²⁺, a pump for pumping a fluid containing oxygen (O₂) andcarbon dioxide (CO₂) into the cathode compartment; and b) an anodecompartment containing an anode electrode with a fuel having a hydrogenconstituent being pumped into said anode compartment, said anodecompartment being separated from said cathode compartment by a membranepermeable to protons, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+), formed by the oxidation of the fuelcross the membrane into the cathode compartment, and wherein the ferrousions are oxidized by the chemolithotrophic microorganisms to ferric ions(Fe³⁺) in an aerobic oxidation reaction in the cathode compartment givenby 4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O, wherein electrical power is obtained bymaking electrical connection between a load and the anode and cathodeelectrodes.
 37. The bio-fuel cell system according to claim 36 whereinthe bioreactor and the cathode compartment contain dissolved nutrientsfor facilitating growth of the chemolithotrophic microorganisms.
 38. Thebio-fuel cell system according to claim 37 wherein the dissolvednutrients is one or more of ammonium sulfate, potassium phosphate,magnesium sulfate, potassium chloride, calcium nitrate, calcium chlorideand sulfuric acid.
 39. The bio-fuel cell system according to claim 36,wherein the membrane permeable to protons is a proton exchange membrane.40. The bio-fuel cell system according to claim 36, wherein the membranepermeable to protons is made of a substantially inert material havingpores extending therethrough less than about 10 micrometers in diameter.41. The bio-fuel cell system according to claim 36, wherein the fluidcontaining oxygen is in a gaseous form.
 42. The bio-fuel cell systemaccording to claim 36, wherein the fluid containing oxygen (O₂) andcarbon dioxide (CO₂) pumped into the cathode compartment is an aqueoussolution containing oxygen (O₂) and carbon dioxide (CO₂) dissolvedtherein.
 43. The bio-fuel cell system according to claim 36, wherein thefuel having a hydrogen constituent is selected from the group consistingof hydrogen gas, methanol, methane and ethanol.
 44. The bio-fuel cellsystem according to claim 36, wherein the fuel having a hydrogenconstituent is hydrogen gas, and wherein the electrochemical oxidationreaction is oxidation of hydrogen at the anode electrode in a reactiongiven by 2H₂=4H++4e−, and so that an overall bio-fuel cell reaction isgiven by 2H₂+O₂=2H₂O.
 45. The bio-fuel cell system according to claim36, including voltage control means for applying and controlling avoltage on the cathode electrode for controlling a ratio of electricalproduction to biomass production by varying microbial cultivationparameters.
 46. The bio-fuel cell system according to claim 36,including reagent control means for controlling a ratio of Fe²⁺/Fe³⁺concentrations for varying microbial cultivation parameters in order tocontrol a ratio of electrical production to biomass production.
 47. Thebio-fuel cell system according to claim 37 including reagent controlmeans for controlling concentrations of the dissolved nutrientsconcentrations for varying microbial cultivation parameters in order tocontrol a ratio of electrical production to biomass production.
 48. Thebio-fuel cell system according to claim 36, wherein thechemolithotrophic microorganisms are Acidithiobacillus ferroxidans. 49.The bio-fuel cell system according to claim 36, wherein thechemolithotrophic microorganisms are selected from the group consistingof Leptospirillum ferrooxidans, Acidimicrobium, Alicyclobacillus, andSulfobacillus.
 50. The bio-fuel cell system according to claim 36,wherein the cathode electrode is made from a chemically inertelectrically conducting material.
 51. The bio-fuel cell system accordingto claim 50 wherein the cathode electrode includes a layer of a porousmaterial selected from the group consisting of carbon, nickel andstainless steel.
 52. The bio-fuel cell system according to claim 50wherein the cathode electrode includes a solid plate of a materialselected from the group consisting of carbon, nickel and stainlesssteel.
 53. A bio-fuel cell system, comprising; a) a cathode compartmentcontaining a cathode electrode, chemolithotrophic microorganisms beingimmobilized on the cathode and in contact with an aqueous solutioncontaining a salt of iron, with a reaction at the cathode electrodebeing reduction of ferric ions at the cathode electrode in a reactiongiven by 4Fe³⁺+4e⁻=4Fe²⁺, a pump for pumping a fluid containing oxygen(O₂) and carbon dioxide into the cathode compartment; and b) an anodecompartment containing an anode electrode with a fuel having a hydrogenconstituent being pumped into said anode compartment, said anodecompartment being separated from said cathode compartment by a membranepermeable to protons, a reaction at the anode electrode beingelectrochemical oxidation of the fuel to produce electrons (e⁻) andprotons (H⁺), wherein protons (H+) formed by the oxidation of the fuelcross the membrane into the cathode compartment, and wherein the ferrousions (Fe²⁺) are oxidized by the chemolithotrophic microorganisms toferric ions (Fe³⁺) in an aerobic oxidation reaction in the cathodecompartment given by 4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O, wherein electrical poweris obtained by making electrical connection between a load and the anodeand cathode electrodes.
 54. The bio-fuel cell system according to claim53 wherein the aqueous solution containing a salt of iron in contactwith the chemolithotrophic microorganisms is a capillary layer coatingthe chemolithotrophic microorganisms and cathode, and wherein the fluidcontaining oxygen (O₂) and carbon dioxide pumped into the cathodecompartment is an oxygen-containing gas.
 55. The bio-fuel cell systemaccording to claim 53 wherein the fluid containing oxygen (O₂) pumpedinto the cathode compartment is the aqueous solution containing the saltof iron which has oxygen (O₂) and carbon dioxide dissolved therein. 56.The bio-fuel cell system according to claim 53, wherein the fuel havinga hydrogen constituent is selected from the group consisting of hydrogengas, methanol, methane and ethanol.
 57. The bio-fuel cell systemaccording to claim 56 wherein the fuel having a hydrogen constituent ishydrogen gas, and wherein the electrochemical oxidation reaction isoxidation of hydrogen at the anode electrode in a reaction given by2H₂=4H+4e−, and so that an overall bio-fuel cell reaction is given by2H₂+O₂=2H₂O.
 58. The bio-fuel cell system according to claim 53,including voltage control means for applying and controlling a voltageon the cathode electrode for controlling a ratio of electricalproduction to biomass production by varying microbial cultivationparameters.
 59. The bio-fuel cell system according to claim 53, whereinthe chemolithotrophic microorganisms are Acidithiobacillus ferroxidans.60. The bio-fuel cell system according to claim 53, wherein thechemolithotrophic microorganisms are selected from the group consistingof Leptospirillum ferrooxidans, Acidimicrobium, Alicyclobacillus, andSulfobacillus.
 61. The bio-fuel cell system according to claim claim 53wherein the cathode electrode is made from a chemically inertelectrically conducting material.
 62. A method for generatingelectricity, comprising; a) pumping a fluid containing oxygen and carbondioxide into a cathode compartment of a fuel cell, the cathodecompartment including a cathode electrode and a redox couple presenttherein with a reaction at the cathode electrode being reduction of afirst member of the redox couple to a second member of the redox couplein a lower oxidation state; b) pumping fuel into an anode compartment ofthe fuel cell containing an anode electrode with the fuel having ahydrogen constituent, said anode compartment being separated from saidcathode compartment by a proton exchange membrane, a reaction at theanode electrode being electrochemical oxidation of the fuel to produceelectrons (e⁻) and protons (H⁺), wherein protons (H+), formed by theoxidation of the fuel cross the proton exchange membrane into thecathode compartment; and c) oxidizing the second member of the redoxcouple in the lower oxidation state back to the higher oxidation stateby chemolithotrophic microorganisms in the presence of oxygen whereinelectrical power in an electrical load is obtained by making electricalconnection between the electrical load and the anode and cathodeelectrodes.
 63. The method according to claim 62 wherein the redoxcouple is Fe²⁺/Fe³⁺, and the wherein the reaction at the cathodeelectrode is reduction of ferric ions at the cathode electrode in areaction given by 4Fe³⁺+4e⁻=4Fe²⁺, and wherein the chemolithotrophicmicroorganisms are contained in a bioreactor into which a fluidcontaining oxygen (O₂) is pumped into the bioreactor, the bioreactorbeing in flow communication with the cathode compartment so that theaqueous solution containing ferrous ions (Fe²⁺) and protons (H⁺) iscirculated from the cathode compartment to the bioreactor where theferrous ions (Fe²⁺) are oxidized by the chemolithotrophic microorganismsto ferric ions (Fe³⁺) in an aerobic oxidation reaction given by4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O.
 64. The method according to claim 62 whereinthe redox couple is Fe²⁺/Fe³⁺, and wherein the reaction at the cathodeelectrode is reduction of ferric ions at the cathode electrode in areaction given by 4Fe³⁺+4e⁻=4Fe²⁺, and wherein the chemolithotrophicmicroorganisms are contained in the cathode compartment so that theferrous ions (Fe²⁺) are oxidized by the chemolithotrophic microorganismsto ferric ions (Fe³⁺) in an aerobic oxidation reaction given by4Fe²⁺+4H⁺+O₂=4Fe³⁺+2H₂O.
 65. The method according to claim 62, whereinthe chemolithotrophic microorganisms are Acidithiobacillus ferroxidans.66. The method according to claim 62, wherein the chemolithotrophicmicroorganisms are selected from the group consisting of Leptospirillumferrooxidans, Acidimicrobium, Alicyclobacillus, and Sulfobacillus. 67.The method according to claim 62, wherein the fuel having a hydrogenconstituent is selected from the group consisting of hydrogen gas,methanol, methane and ethanol.
 68. The bio-fuel cell system according toclaim 67 wherein the fuel having a hydrogen constituent is hydrogen gas,and wherein the electrochemical oxidation reaction is oxidation ofhydrogen at the anode electrode in a reaction given by 2H₂=4H++4e−, andso that an overall bio-fuel cell reaction is given by 2H₂+O₂=2H₂O. 69.The method according to claim 62, including controlling a ratio ofelectrical production to biomass production by varying microbialcultivation parameters including an electrical potential of the cathodeelectrode, or by varying the ratio of Fe²⁺/Fe³⁺ concentrations, or acombination of both.